Electrodes, electrochemical cells, and methods of forming electrodes and electrochemical cells

ABSTRACT

Electrodes and methods of forming electrodes are described herein. The electrode can be an electrode of an electrochemical cell or battery. The electrode includes a current collector and a film in electrical communication with the current collector. The film may include a carbon phase that holds the film together. The electrode further includes an electrode attachment substance that adheres the film to the current collector.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/522,093, filed Nov. 9, 2021, which is a continuation of U.S.application Ser. No. 16/694,849, filed Nov. 25, 2019, which is acontinuation of U.S. application Ser. No. 15/788,564, filed Oct. 19,2017, which is a continuation of U.S. application Ser. No. 14/302,321,filed Jun. 11, 2014, which is a divisional of U.S. application Ser. No.13/796,922, filed Mar. 12, 2013, which is a continuation-in-part of U.S.application Ser. No. 13/333,864, filed Dec. 21, 2011, which claims thebenefit of U.S. Provisional Application Nos. 61/426,446, filed Dec. 22,2010, and 61/488,313, filed May 20, 2011, the entirety of each of whichis hereby incorporated by reference.

BACKGROUND Field of the Invention

The present disclosure relates to electrochemical cells and electrodesused in electrochemical cells. In particular, the present disclosurerelates to electrodes and electrochemical cells that include silicon andcarbon composite materials for use in batteries.

Description of the Related Art

A lithium ion battery typically includes a separator and/or electrolytebetween an anode and a cathode. In one class of batteries, theseparator, cathode and anode materials are individually formed intosheets or films. Sheets of the cathode, separator and anode aresubsequently stacked or rolled with the separator separating the cathodeand anode (e.g., electrodes) to form the battery. For the cathode,separator and anode to be rolled, each sheet must be sufficientlydeformable or flexible to be rolled without failures, such as cracks,brakes, mechanical failures, etc. Typical electrodes includeelectro-chemically active material layers on electrically conductivemetals (e.g., aluminum and copper). For example, carbon can be depositedonto a current collector along with an inactive binder material. Carbonis often used because it has excellent electrochemical properties and isalso electrically conductive. Electrodes can be rolled or cut intopieces which are then layered into stacks. The stacks are of alternatingelectro-chemically active materials with the separator between them.

SUMMARY

In certain embodiments, an electrode is provided. The electrode caninclude a current collector and a film in electrical communication withthe current collector. The film may include a carbon phase that holdsthe film together. The electrode may also include an electrodeattachment substance that adheres the film to the current collector.

The film may be a monolithic self-supporting film. Furthermore, the filmmay include silicon particles distributed within the carbon phase. Thecarbon phase may include hard carbon. Furthermore, the film may includeporosity and at least some of the electrode attachment substance may bewithin the porosity of the film. For example, the porosity can be about5 to about 50 percent by volume of the film and/or about 1 to about 70percent by volume of the film.

The electrical attachment substance may include a polymer such aspolyamideimide, polyvinylidene fluoride, and polyacrylic acid.Furthermore, electrode attachment substance can be substantiallyelectrically nonconductive. The electrode attachment substance may allowfor expansion of the anode active material and current collector withoutsignificant failure of the electrode. For example, the electrode may beable to be bent to a radius of curvature of at least 7 mm withoutsignificant cracking.

In certain embodiments, a method of forming an electrode is provided.The method may include sandwiching an electrode attachment substancebetween a current collector and a solid film comprisingelectrochemically active material such that the electrode attachmentsubstance adheres the solid film to the current collector and the solidfilm is in electrical communication with the current collector. In someembodiments, the solid film at least partially absorbs the electrodeattachment substance into porosity of the film.

In certain embodiments, an electrochemical cell is provided. Theelectrochemical cell may include a porous separator sheet and a cellattachment substance sandwiched between the porous separator sheet andthe electrode described above. The cell attachment substance can includepolyvinylidene fluoride. The cell attachment substance may coat at leastone of or both of the porous separator sheet and the electrode. Forexample, the cell attachment substance that coats the porous separatorsheet can be a first cell attachment substance and the cell attachmentsubstance that coats the electrode can be a second cell attachmentsubstance that is chemically different than the first cell attachmentsubstance.

In certain embodiments, a method of forming an electrochemical cell isprovided. The method can include sandwiching a cell attachment substancebetween a porous separator sheet and the electrode described above. Themethod may further include coating at least one of or both of the porousseparator sheet and the electrode with the cell attachment substance.Moreover, the method may include heating the cell attachment substanceafter sandwiching the cell attachment substance between the porousseparator sheet and the electrode.

In certain embodiments, an electrode is provided. The electrode caninclude a current collector and a film in electrical communication withthe current collector. The film may include a carbon phase that holdsthe film together. The electrode may also include an electrodeattachment substance that adheres the film to the current collector. Thefilm may include porosity and at least about 90 percent of the porositymay be substantially free of the electrode attachment substance.

The electrode attachment substance may be substantially electricallynonconductive. Furthermore, the electrode attachment substance may forma substantially uniform layer disposed substantially over an entiresurface of the film. The electrode attachment substance may include apolymer not soluble in a nonaqueous electrolyte solution. In someembodiments, the nonaqueous electrolyte solution includes a carbonatesolvent. The polymer can include polyamideimide, polyvinylidenefluoride, polyethylene, or polypropylene. The current collector caninclude copper.

In some embodiments, the electrode may further include a secondelectrode attachment substance sandwiched between the current collectorand a second film in electrical communication with the currentcollector. The film may include an anode. The anode may include silicon.The film may include porosity. For example, the porosity can be about 5to about 50 percent by volume of the film or about 1 to about 70 percentby volume of the film. The film may have surfaces that are substantiallyfree of the electrode attachment substance.

In certain embodiments, a method of forming an electrode is provided.The method may include providing a current collector coated with a firstelectrode attachment substance on a first side of the current collector.The first electrode attachment substance may be in a substantially solidstate. The method may also include disposing a first solid filmcomprising electrochemically active material on the first electrodeattachment substance; and heating the first electrode attachmentsubstance to adhere the first solid film to the current collector.

The method can further include providing a second electrode attachmentsubstance on a second side of the current collector. The secondelectrode attachment substance may be in a substantially solid state.Furthermore, the method can include disposing a second solid filmcomprising electrochemically active material on the second electrodeattachment substance; and heating the second electrode attachmentsubstance to adhere the second solid film to the current collector.Heating the first electrode attachment substance and heating the secondelectrode attachment substance may occur simultaneously.

In some embodiments, providing a current collector may include coatingthe current collector with a polymer solution on the first side of thecurrent collector; and drying the polymer solution to form the firstelectrode attachment substance. Providing a second electrode attachmentsubstance may include coating the current collector with a polymersolution on the second side of the current collector; and drying thepolymer solution to form the second electrode attachment substance.

In other embodiments, providing a current collector may includeproviding a polymer resin on the first side of the current collector;and extrusion coating the polymer resin to form the first electrodeattachment substance. Providing a second electrode attachment substancemay include providing a polymer resin on the second side of the currentcollector; and extrusion coating the polymer resin to form the secondelectrode attachment substance.

In some embodiments of the method, the first electrode attachmentsubstance includes a polymer that is not soluble in a nonaqueouselectrolyte solution. The nonaqueous electrolyte solution can include acarbonate solvent. The polymer can include polyamideimide,polyvinylidene fluoride, polyethylene, or polypropylene. In certainembodiments of the method, heating includes heat laminating, rollpressing, or flat pressing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of a method of forming a compositematerial that includes forming a mixture that includes a precursor,casting the mixture, drying the mixture, curing the mixture, andpyrolyzing the precursor;

FIG. 2 is a plot of the discharge capacity at an average rate of C/2.6;

FIG. 3 is a plot of the discharge capacity at an average rate of C/3;

FIG. 4 is a plot of the discharge capacity at an average rate of C/3.3;

FIG. 5 is a plot of the discharge capacity at an average rate of C/5;

FIG. 6 is a plot of the discharge capacity at an average rate of C/9;

FIG. 7 is a plot of the discharge capacity;

FIG. 8 is a plot of the discharge capacity at an average rate of C/9;

FIGS. 9A and 9B are plots of the reversible and irreversible capacity asa function of the various weight percentage of PI derived carbon from2611c and graphite particles for a fixed percentage of 20 wt. % Si;

FIG. 10 is a plot of the first cycle discharge capacity as a function ofweight percentage of carbon;

FIG. 11 is a plot of the reversible (discharge) and irreversiblecapacity as a function of pyrolysis temperature;

FIG. 12 is a photograph of a 4.3 cm×4.3 cm composite anode film withouta metal foil support layer;

FIG. 13 is a scanning electron microscope (SEM) micrograph of acomposite anode film before being cycled (the out-of-focus portion is abottom portion of the anode and the portion that is in focus is acleaved edge of the composite film);

FIG. 14 is another SEM micrograph of a composite anode film before beingcycled;

FIG. 15 is a SEM micrograph of a composite anode film after being cycled10 cycles;

FIG. 16 is another SEM micrograph of a composite anode film after beingcycled 10 cycles;

FIG. 17 is a SEM micrograph of a composite anode film after being cycled300 cycles;

FIG. 18 includes SEM micrographs of cross-sections of composite anodefilms;

FIG. 19 is a photograph of composite film showing wrinkles formed in thefilm as a result of cycling;

FIG. 20 is a photograph of a composite film without an electrodeattachment substance showing disintegration of the film as a result ofcycling;

FIG. 21 is a photograph of a composite film with an electrode attachmentsubstance of polyvinylidene fluoride (PVDF);

FIG. 22 is a photograph of a composite film with an electrode attachmentsubstance of polyamideimide (PAI);

FIG. 23 is a plot of gravimetric discharge capacity density as afunction of cycles for samples with different electrode attachmentsubstances and without an electrode attachment substance;

FIG. 24 is a plot of discharge capacity as percentage of 8th dischargecapacity as a function of cycles for samples with an electrodeattachment substance of PAI and without an electrode attachmentsubstance at an average charge rate of C and average discharge rate ofC;

FIG. 25 is a plot of discharge capacity as percentage of 8th dischargecapacity as a function of cycles for samples with an electrodeattachment substance of PAI and without an electrode attachmentsubstance at an average charge rate of C/5 and average discharge rate ofC/2;

FIG. 26 is a plot of discharge capacity as percentage of 2nd dischargecapacity as a function of cycles for samples with an electrodeattachment substance of PAI and samples with an electrode attachmentsubstance of PVDF at an average charge rate of C/5 and average dischargerate of C/5;

FIGS. 27A-D are illustrations of an example method of assembling anelectrode stack for heat lamination.

FIG. 28 is a bar graph comparing the average irreversible capacity forelectrode assemblies formed by different methods of attaching compositefilms to the current collector.

FIG. 29 is a plot of discharge capacity as a function of number ofcycles for cells with a cell attachment substance and cells without acell attachment substance;

FIG. 30 is a plot of discharge capacity as a function of number ofcycles for cells with a cell attachment substance comparing samples withdifferent separator materials;

FIG. 31 is a plot of discharge capacity as a function of number ofcycles for cells with a cell attachment substance comparing samples withdifferent electrolytes;

FIG. 32 is a photograph of an electrode showing wrinkling of an anodefilm;

FIG. 33A-C are photographs of anode films where pressure of (A) 100 lb,(B) 75 lb, and (C) 50 lb was applied to the cell; and

FIG. 34 is a photograph of an anode film which shows the absence ofwrinkles.

DETAILED DESCRIPTION

This application describes certain embodiments of electrodes (e.g.,anodes and cathodes), electrochemical cells, and methods of formingelectrodes and electrochemical cells that may include a carbonizedpolymer. For example, a mixture that includes a precursor includingsilicon can be formed into a silicon composite material. This mixtureincludes both carbon and silicon and thus can be referred to as asilicon composite material as well as a carbon composite material.Examples of mixtures and carbon composite materials and carbon-siliconcomposite materials that can be used in certain electrodes, cells, andmethods described below are described in U.S. patent application Ser.No. 13/008,800, filed Jan. 18, 2011, and published on Jul. 21, 2011 asU.S. Patent Application Publication No. 2011/0177393, entitled“Composite Materials for Electrochemical Storage,” the entirety of whichis hereby incorporated by reference. In addition, certain embodiments ofmethods of forming an electrode and/or electrochemical cell using anattachment substance between a composite film and a current collectorand/or between an electrode and a separator are also disclosed. Methodsof reducing wrinkling of anodes are also provided.

I. Composite Materials

Typical carbon anode electrodes include a current collector such as acopper sheet. Carbon is deposited onto the collector along with aninactive binder material. Carbon is often used because it has excellentelectrochemical properties and is also electrically conductive. If thecurrent collector layer (e.g., copper layer) was removed, the carbonwould be unable to mechanically support itself. Therefore, conventionalelectrodes require a support structure such as the collector to be ableto function as an electrode. The electrode (e.g., anode or cathode)compositions described in this application can produce electrodes thatare self-supported. The need for a metal foil current collector iseliminated or minimized because conductive carbonized polymer is usedfor current collection in the anode structure as well as for providingmechanical support. A current collector may be preferred in someapplications where current above a certain threshold is required.Methods of attachment of the composite film (e.g., piece) to a currentcollector are described in section II below. The carbonized polymer canform a substantially continuous conductive carbon phase in the entireelectrode as opposed to particulate carbon suspended in a non-conductivebinder in one class of conventional lithium-ion battery electrodes.Advantages of a carbon composite blend that utilizes a carbonizedpolymer can include, for example, 1) higher capacity, 2) enhancedovercharge/discharge protection, 3) lower irreversible capacity due tothe elimination (or minimization) of metal foil current collectors, and4) potential cost savings due to simpler manufacturing.

Anode electrodes currently used in the rechargeable lithium-ion cellstypically have a specific capacity of approximately 200 milliamp hoursper gram (including the metal foil current collector, conductiveadditives, and binder material). Graphite, the active material used inmost lithium ion battery anodes, has a theoretical energy density of 372milliamp hours per gram (mAh/g). In comparison, silicon has a hightheoretical capacity of 4200 mAh/g. Silicon, however, swells in excessof 300% upon lithiation. Because of this expansion, anodes includingsilicon may expand/contract and lose electrical contact to the rest ofthe anode. Therefore, a silicon anode should be designed to be able toexpand while maintaining good electrical contact with the rest of theelectrode.

This application also describes certain embodiments of a method ofcreating monolithic, self-supported anodes using a carbonized polymer.Because the polymer is converted into a electrically conductive andelectrochemically active matrix, the resulting electrode is conductiveenough that a metal foil or mesh current collector can be omitted orminimized. The converted polymer also may act as an expansion buffer forsilicon particles during cycling so that a high cycle life can beachieved. In certain embodiments, the resulting electrode is anelectrode that is comprised substantially of active material. In furtherembodiments, the resulting electrode is substantially active material.The electrodes can have a high energy density of between about 500 mAh/gto about 1200 mAh/g that can be due to, for example, 1) the use ofsilicon, 2) elimination or substantial reduction of metal currentcollectors, and 3) being comprised entirely (or almost entirely) ofactive material.

The composite materials described herein can be used as an anode in mostconventional lithium ion batteries; they may also be used as the cathodein some electrochemical couples with additional additives. The compositematerials can also be used in either secondary batteries (e.g.,rechargeable) or primary batteries (e.g., non-rechargeable). In certainembodiments, the composite materials are self-supported structures. Infurther embodiments, the composite materials are self-supportedmonolithic structures. For example, a collector may not be included inthe electrode comprised of the composite material. In certainembodiments, the composite material can be used to form carbonstructures discussed in U.S. Patent Application Publication No.2011/0020701 entitled “Carbon Electrode Structures for Batteries,” theentirety of which is hereby incorporated by reference. Furthermore, thecomposite materials described herein can be, for example, siliconcomposite materials, carbon composite materials, and/or silicon-carboncomposite materials.

FIG. 1 illustrates one embodiment of a method of forming a compositematerial 100. For example, the method of forming a composite materialcan include forming a mixture including a precursor, block 101. Themethod can further include pyrolyzing the precursor to convert theprecursor to a carbon phase. The precursor mixture may include carbonadditives such as graphite active material, chopped or milled carbonfiber, carbon nanofibers, carbon nanotubes, and/or other carbons. Afterthe precursor is pyrolyzed, the resulting carbon material can be aself-supporting monolithic structure. In certain embodiments, one ormore materials are added to the mixture to form a composite material.For example, silicon particles can be added to the mixture. Thecarbonized precursor results in an electrochemically active structurethat holds the composite material together. For example, the carbonizedprecursor can be a substantially continuous phase. The silicon particlesmay be distributed throughout the composite material. Advantageously,the carbonized precursor will be a structural material as well as anelectro-chemically active and electrically conductive material. Incertain embodiments, material particles added to the mixture arehomogenously distributed throughout the composite material to form ahomogeneous composite.

The mixture can include a variety of different components. The mixturecan include one or more precursors. In certain embodiments, theprecursor is a hydrocarbon compound. For example, the precursor caninclude polyamic acid, polyimide, etc. Other precursors include phenolicresins, epoxy resins, and other polymers. The mixture can furtherinclude a solvent. For example, the solvent can be N-methyl-pyrollidone(NMP). Other possible solvents include acetone, diethyl ether, gammabutyrolactone, isopropanol, dimethyl carbonate, ethyl carbonate,dimethoxyethane, etc. Examples of precursor and solvent solutionsinclude PI-2611 (HD Microsystems), PI-5878G (HD Microsystems) and VTECPI-1388 (RBI, Inc.). PI-2611 is comprised of >60% n-methyl-2-pyrollidoneand 10-30% s-biphenyldianhydride/p-phenylenediamine. PI-5878G iscomprised of >60% n-methylpyrrolidone, 10-30% polyamic acid ofpyromellitic dianhydride/oxydianiline, 10-30% aromatic hydrocarbon(petroleum distillate) including 5-10% 1,2,4-trimethylbenzene. Incertain embodiments, the amount of precursor (e.g., solid polymer) inthe solvent is about 10 wt. % to about 30 wt. %. Additional materialscan also be included in the mixture. For example, as previouslydiscussed, silicon particles or carbon particles including graphiteactive material, chopped or milled carbon fiber, carbon nanofibers,carbon nanotubes, and other conductive carbons can be added to themixture. In addition, the mixture can be mixed to homogenize themixture.

In certain embodiments, the mixture is cast on a substrate, block 102 inFIG. 1 . In some embodiments, casting includes using a gap extrusion ora blade casting technique. The blade casting technique can includeapplying a coating to the substrate by using a flat surface (e.g.,blade) which is controlled to be a certain distance above the substrate.A liquid or slurry can be applied to the substrate, and the blade can bepassed over the liquid to spread the liquid over the substrate. Thethickness of the coating can be controlled by the gap between the bladeand the substrate since the liquid passes through the gap. As the liquidpasses through the gap, excess liquid can also be scraped off. Forexample, the mixture can be cast on a polymer sheet, a polymer roll, orfoils or rolls made of glass or metal. The mixture can then be dried toremove the solvent, block 103. For example, a polyamic acid and NMPsolution can be dried at about 110° C. for about 2 hours to remove theNMP solution. The dried mixture can then be removed from the substrate.For example, an aluminum substrate can be etched away with HCl.Alternatively, the dried mixture can be removed from the substrate bypeeling or otherwise mechanically removing the dried mixture from thesubstrate. In certain embodiments, the dried mixture is a precursor filmor sheet. In some embodiments, the dried mixture is cured, block 104. Ahot press can be used to cure and to keep the dried mixture flat. Forexample, the dried mixture from a polyamic acid and NMP solution can behot pressed at about 200° C. for about 8 to 16 hours. Alternatively, theentire process including casting and drying can be done as aroll-to-roll process using standard film-handling equipment. The driedmixture can be rinsed to remove any solvents or etchants that mayremain. For example, de-ionized (DI) water can be used to rinse thedried mixture. In certain embodiments, tape casting techniques can beused for the casting. In other embodiments, there is no substrate forcasting and the anode film does not need to be removed from anysubstrate. The dried mixture may be cut or mechanically sectioned intosmaller pieces.

The mixture further goes through pyrolysis to convert the precursor tocarbon, block 105. In certain embodiments, the mixture is pyrolysed in areducing atmosphere. For example, an inert atmosphere, a vacuum and/orflowing argon, nitrogen, or helium gas can be used. In some embodiments,the mixture is heated to about 900° C. to about 1350° C. For example,polyimide formed from polyamic acid can be carbonized at about 1175° C.for about one hour. In certain embodiments, the heat up rate and/or cooldown rate of the mixture is about 10° C./min. A holder may be used tokeep the mixture in a particular geometry. The holder can be graphite,metal, etc. In certain embodiments, the mixture is held flat. After themixture is pyrolysed, tabs can be attached to the pyrolysed material toform electrical contacts. For example, nickel, copper or alloys thereofcan be used for the tabs.

In certain embodiments, one or more of the methods described herein is acontinuous process. For example, casting, drying, curing and pyrolysiscan be performed in a continuous process; e.g., the mixture can becoated onto a glass or metal cylinder. The mixture can be dried whilerotating on the cylinder creating a film. The film can be transferred asa roll or peeled and fed into another machine for further processing.Extrusion and other film manufacturing techniques known in industrycould also be utilized prior to the pyrolysis step.

Pyrolysis of the precursor results in a carbon material (e.g., at leastone carbon phase). In certain embodiments, the carbon material is a hardcarbon. In some embodiments, the precursor is any material that can bepyrolysed to form a hard carbon. When the mixture includes one or moreadditional materials or phases in addition to the carbonized precursor,a composite material can be created. In particular, the mixture caninclude silicon particles creating a silicon-carbon (e.g., at least onefirst phase comprising silicon and at least one second phase comprisingcarbon) or silicon-carbon-carbon (e.g., at least one first phasecomprising silicon, at least one second phase comprising carbon, and atleast one third phase comprising carbon) composite material. Siliconparticles can increase the specific lithium insertion capacity of thecomposite material. When silicon absorbs lithium ions, it experiences alarge volume increase on the order of 300+ volume percent which cancause electrode structural integrity issues. In addition to volumetricexpansion related problems, silicon is not inherently electricallyconductive, but becomes conductive when it is alloyed with lithium(e.g., lithiation). When silicon de-lithiates, the surface of thesilicon losses electrical conductivity. Furthermore, when siliconde-lithiates, the volume decreases which results in the possibility ofthe silicon particle losing contact with the matrix. The dramatic changein volume also results in mechanical failure of the silicon particlestructure, in turn, causing it to pulverize. Pulverization and loss ofelectrical contact have made it a challenge to use silicon as an activematerial in lithium-ion batteries. A reduction in the initial size ofthe silicon particles can prevent further pulverization of the siliconpowder as well as minimizing the loss of surface electricalconductivity. Furthermore, adding material to the composite that canelastically deform with the change in volume of the silicon particlescan ensure that electrical contact to the surface of the silicon is notlost. For example, the composite material can include carbons such asgraphite which contributes to the ability of the composite to absorbexpansion and which is also capable of intercalating lithium ions addingto the storage capacity of the electrode (e.g., chemically active).Therefore, the composite material may include one or more types ofcarbon phases.

Embodiments of a largest dimension of the silicon particles includesless than about 40 μm, less than about 1 μm, between about 10 nm and 40μm, between about 10 nm and 1 μm, less than about 500 nm, less thanabout 100 nm, and about 100 nm. All, substantially all, or at least someof the silicon particles may comprise the largest dimension describedabove. For example, an average or median largest dimension of thesilicon particles include less than about 40 μm, less than about 1 μm,between about 10 nm and 40 μm, between about 10 nm and 1 μm, less thanabout 500 nm, less than about 100 nm, and about 100 nm. The amount ofsilicon in the composite material can be greater than zero percent byweight of the mixture and composite material. In certain embodiments,the amount of silicon in the mixture is between greater than 0% and lessthan about 90% by weight or between about 30% and about 80% by weight ofthe mixture. Embodiments of the amount of silicon in the compositematerial include greater than 0% and less than about 35% by weight,greater than 0% and less than about 25% by weight, between about 10 andabout 35% by weight, and about 20% by weight. In further certainembodiments, the amount of silicon in the mixture is at least about 30%by weight. Additional embodiments of the amount of silicon in thecomposite material include more than about 50% by weight, between about30% and about 80% by weight, between about 50% and about 70% by weight,and between about 60% and about 80% by weight. Furthermore, the siliconparticles may or may not be pure silicon. For example, the siliconparticles may be substantially silicon or may be a silicon alloy. In oneembodiment, the silicon alloy includes silicon as the primaryconstituent along with one or more other elements.

The amount of carbon obtained from the precursor can be about 50 weightpercent from polyamic acid. In certain embodiments, the amount of carbonfrom the precursor in the composite material is about 10 to 25% byweight. The carbon from the precursor can be hard carbon. Hard carbon isa carbon that does not convert into graphite even with heating in excessof 2800 degrees Celsius. Precursors that melt or flow during pyrolysisconvert into soft carbons and/or graphite with sufficient temperatureand/or pressure. Hard carbon may be selected since soft carbonprecursors may flow and soft carbons and graphite are mechanicallyweaker than hard carbons. Other possible hard carbon precursors includephenolic resins, epoxy resins, and other polymers that have a very highmelting point or are crosslinked. Embodiments of the amount of hardcarbon in the composite material includes about 10% to about 25% byweight, about 20% by weight, and more than about 50% by weight. Incertain embodiments, the hard carbon phase is substantially amorphous.In other embodiments, the hard carbon phase is substantiallycrystalline. In further embodiments, the hard carbon phase includesamorphous and crystalline carbon. The hard carbon phase can be a matrixphase in the composite material. The hard carbon can also be embedded inthe pores of the additives including silicon. The hard carbon may reactwith some of the additives to create some materials at interfaces. Forexample, there may be a silicon carbide layer between silicon particlesand the hard carbon.

In certain embodiments, graphite particles are added to the mixture.Advantageously, graphite is an electrochemically active material in thebattery as well as an elastic deformable material that can respond tovolume change of the silicon particles. Graphite is the preferred activeanode material for certain classes of lithium-ion batteries currently onthe market because it has a low irreversible capacity. Additionally,graphite is softer than hard carbon and can better absorb the volumeexpansion of silicon additives. In certain embodiments, the largestdimension of the graphite particles is between about 0.5 microns andabout 20 microns. All, substantially all, or at least some of thegraphite particles may comprise the largest dimension described herein.In further embodiments, the average or median largest dimension of thegraphite particles is between about 0.5 microns and about 20 microns. Incertain embodiments, the mixture includes greater than 0% and less thanabout 80% by weight graphite particles. In further embodiments, thecomposite material includes about 40% to about 75% by weight graphiteparticles.

In certain embodiments, conductive particles which may also beelectrochemically active are added to the mixture. Such particlesprovide both a more electronically conductive composite as well as amore mechanically deformable composite capable of absorbing the largevolumetric change incurred during lithiation and de-lithiation. Incertain embodiments, the largest dimension of the conductive particlesis between about 10 nanometers and about 7 millimeters. All,substantially all, or at least some of the conductive particles maycomprise the largest dimension described herein. In further embodiments,the average or median largest dimension of the conductive particles isbetween about 10 nm and about 7 millimeters. In certain embodiments, themixture includes greater than zero and up to about 80% by weightconductive particles. In further embodiments, the composite materialincludes about 45% to about 80% by weight conductive particles. Theconductive particles can be conductive carbon including carbon blacks,carbon fibers, carbon nanofibers, carbon nanotubes, etc. Many carbonsthat are considered as conductive additives that are notelectrochemically active become active once pyrolyzed in a polymermatrix. Alternatively, the conductive particles can be metals or alloysincluding copper, nickel, or stainless steel.

In certain embodiments, an electrode can include a composite materialdescribed herein. For example, a composite material can form aself-supported monolithic electrode. The pyrolyzed carbon phase (e.g.,hard carbon phase) of the composite material can hold together andstructurally support the particles that were added to the mixture. Incertain embodiments, the self-supported monolithic electrode does notinclude a separate collector layer and/or other supportive structures.In some embodiments, the composite material and/or electrode does notinclude a polymer beyond trace amounts that remain after pyrolysis ofthe precursor. In further embodiments, the composite material and/orelectrode does not include a non-electrically conductive binder.

In some embodiments, the composite material may also include porosity.For example, the porosity can be about 5% to about 40% by volumeporosity. In some embodiments, the composite material (or the film) caninclude porosity of about 1% to about 70% or about 5% to about 50% byvolume porosity.

In certain embodiments, an electrode in a battery or electrochemicalcell can include a composite material described herein. For example, thecomposite material can be used for the anode and/or cathode. In certainembodiments, the battery is a lithium ion battery. In furtherembodiments, the battery is a secondary battery, or in otherembodiments, the battery is a primary battery.

Furthermore, the full capacity of the composite material may not beutilized during use of battery to improve life of the battery (e.g.,number charge and discharge cycles before the battery fails or theperformance of the battery decreases below a usability level). Forexample, a composite material with about 70% by weight siliconparticles, about 20% by weight carbon from a precursor, and about 10% byweight graphite may have a maximum gravimetric capacity of about 2000mAh/g, while the composite material may only be used up to angravimetric capacity of about 550 to about 850 mAh/g. Although, themaximum gravimetric capacity of the composite material may not beutilized, using the composite material at a lower capacity can stillachieve a higher capacity than certain lithium ion batteries. In certainembodiments, the composite material is used or only used at angravimetric capacity below about 70% of the composite material's maximumgravimetric capacity. For example, the composite material is not used atan gravimetric capacity above about 70% of the composite material'smaximum gravimetric capacity. In further embodiments, the compositematerial is used or only used at an gravimetric capacity below about 50%of the composite material's maximum gravimetric capacity or below about30% of the composite material's maximum gravimetric capacity.

EXAMPLES

The below example processes for anode fabrication generally includemixing components together, casting those components onto a removablesubstrate, drying, curing, removing the substrate, then pyrolyzing theresulting samples. N-Methyl-2-pyrrolidone (NMP) was typically used as asolvent to modify the viscosity of any mixture and render it castableusing a doctor blade approach.

Example 1

In Example 1, a polyimide liquid precursor (PI 2611 from HD Microsystemscorp.), graphite particles (SLP30 from Timcal corp.), conductive carbonparticles (Super P from Timcal corp.), and silicon particles (from AlfaAesar corp.) were mixed together for 5 minutes using a Spex 8000Dmachine in the weight ratio of 200:55:5:20. The mixture was then castonto aluminum foil and allowed to dry in a 90° C. oven, to drive awaysolvents, e.g., NMP. This is followed by a curing step at 200° C. in ahot press, under negligible pressure, for at least 12 hours. Thealuminum foil backing was then removed by etching in a 12.5% HClsolution. The remaining film was then rinsed in DI water, dried and thenpyrolyzed around an hour at 1175° C. under argon flow. The processresulted in a composition of 15.8% of PI 2611 derived carbon, 57.9% ofgraphite particles, 5.3% of carbon resulting from Super P, and 21.1% ofsilicon by weight.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC oxide cathode. A typical cycling graph is shown inFIG. 2 .

Example 2

In Example 2, silicon particles (from EVNANO Advanced Chemical MaterialsCo., Ltd.) were initially mixed with NMP using a Turbula mixer for aduration of one hour at a 1:9 weight ratio. Polyimide liquid precursor(PI 2611 from HD Microsystems corp.), graphite particles (SLP30 fromTimcal corp.), and carbon nanofibers (CNF from Pyrograf corp.) were thenadded to the Si:NMP mixture in the weight ratio of 200:55:5:200 andvortexed for around 2 minutes. The mixture was then cast onto aluminumfoil that was covered by a 21 μm thick copper mesh. The samples werethen allowed to dry in a 90° C. oven to drive away solvents, e.g., NMP.This was followed by a curing step at 200° C. in a hot press, undernegligible pressure, for at least 12 hours. The aluminum foil backingwas then removed by etching in a 12.5% HCl solution. The remaining filmwas then rinsed in DI water, dried and then pyrolyzed for around an hourat 1000° C. under argon. The process resulted in a composition of 15.8%of PI 2611 derived carbon, 57.9% of graphite particles, 5.3% of CNF, and21.1% of silicon by weight.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC oxide cathode. A typical cycling graph is shown inFIG. 3 .

Example 3

In Example 3, polyimide liquid precursor (PI 2611 from HD Microsystemscorp.), and 325 mesh silicon particles (from Alfa Aesar corp.) weremixed together using a Turbula mixer for a duration of 1 hour in theweight ratios of 40:1. The mixture was then cast onto aluminum foil andallowed to dry in a 90° C. oven to drive away solvents, e.g., NMP. Thiswas followed by a curing step at 200° C. in a hot press, undernegligible pressure, for at least 12 hours. The aluminum foil backingwas then removed by etching in a 12.5% HCl solution. The remaining filmwas then rinsed in DI water, dried and then pyrolyzed around an hour at1175° C. under argon flow. The process resulted in a composition of 75%of PI 2611 derived carbon and 25% of silicon by weight.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC Oxide cathode. A typical cycling graph is shown inFIG. 4 .

Example 4

In Example 4, silicon microparticles (from Alfa Aesar corp.), polyimideliquid precursor (PI 2611 from HD Microsystems corp.), graphiteparticles (SLP30 from Timcal corp.), milled carbon fibers (from FibreGlast Developments corp.), carbon nanofibers (CNF from Pyrograf corp.),carbon nanotubes (from CNANO Technology Limited), conductive carbonparticles (Super P from Timcal corp.), conductive graphite particles(KS6 from Timca corp.) were mixed in the weight ratio of20:200:30:8:4:2:1:15 using a vortexer for 5 minutes. The mixture wasthen cast onto aluminum foil. The samples were then allowed to dry in a90° C. oven to drive away solvents, e.g., NMP. This was followed by acuring step at 200° C. in a hot press, under negligible pressure, for atleast 12 hours. The aluminum foil backing was then removed by etching ina 12.5% HCl solution. The remaining film was then rinsed in DI water,dried and then pyrolyzed for around an hour at 1175° C. under argon. Theprocess resulted in a composition similar to the original mixture butwith a PI 2611 derived carbon portion that was 7.5% the original weightof the polyimide precursor.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC oxide cathode. A typical cycling graph is shown inFIG. 5 .

Example 5

In Example 5, polyimide liquid precursor (PI 2611 from HD Microsystemscorp.), and silicon microparticles (from Alfa Aesar corp.) were mixedtogether using a Turbula mixer for a duration of 1 hours in the weightratio of 4:1. The mixture was then cast onto aluminum foil covered witha carbon veil (from Fibre Glast Developments Corporation) and allowed todry in a 90° C. oven to drive away solvents, e.g., NMP. This wasfollowed by a curing step at 200° C. in a hot press, under negligiblepressure, for at least 12 hours. The aluminum foil backing was thenremoved by etching in a 12.5% HCl solution. The remaining film was thenrinsed in DI water, dried and then pyrolyzed around an hour at 1175° C.under argon flow. The process resulted in a composition of approximately23% of PI 2611 derived carbon, 76% of silicon by weight, and the weightof the veil being negligible.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium nickel manganese cobalt oxide (NMC) cathode. A typicalcycling graph is shown in FIG. 6 .

Example 6

In Example 6, polyimide liquid precursor (PI 2611 from HD Microsystemscorp.), graphite particles (SLP30 from Timcal corp.), and siliconmicroparticles (from Alfa Aesar corp.) were mixed together for 5 minutesusing a Spex 8000D machine in the weight ratio of 200:10:70. The mixturewas then cast onto aluminum foil and allowed to dry in a 90° C. oven, todrive away solvents (e.g., NMP). The dried mixture was cured at 200° C.in a hot press, under negligible pressure, for at least 12 hours. Thealuminum foil backing was then removed by etching in a 12.5% HClsolution. The remaining film was then rinsed in DI water, dried and thenpyrolyzed at 1175° C. for about one hour under argon flow. The processresulted in a composition of 15.8% of PI 2611 derived carbon, 10.5% ofgraphite particles, 73.7% of silicon by weight.

The resulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC oxide cathode. The anodes where charged to 600mAh/g each cycle and the discharge capacity per cycle was recorded. Atypical cycling graph is shown in FIG. 7 .

Example 7

In Example 7, PVDF and silicon particles (from EVNANO Advanced ChemicalMaterials Co), conductive carbon particles (Super P from Timcal corp.),conductive graphite particles (KS6 from Timcal corp.), graphiteparticles (SLP30 from Timcal corp.) and NMP were mixed in the weightratio of 5:20:1:4:70:95. The mixture was then cast on a copper substrateand then placed in a 90° C. oven to drive away solvents, e.g., NMP. Theresulting electrodes were then tested in a pouch cell configurationagainst a lithium NMC Oxide cathode. A typical cycling graph is shown inFIG. 8 .

Example 8

Multiple experiments were conducted in order to find the effects ofvarying the percentage of polyimide derive carbon (e.g. 2611c) whiledecreasing the percentage of graphite particles (SLP30 from Timcalcorp.) and keeping the percentage of silicon microparticles (from AlfaAesar corp.) at 20 wt. %.

As shown in FIGS. 9A and 9B, the results show that more graphite andless 2611c was beneficial to cell performance by increasing the specificcapacity while decreasing the irreversible capacity. Minimizing 2611cadversely affected the strength of the resultant anode so a value close20 wt. % can be preferable as a compromise in one embodiment.

Example 9

Similarly to example 8, if 2611c is kept at 20 wt. % and Si percentageis increased at the expense of graphite particles, the first cycledischarge capacity of the resulting electrode is increased. FIG. 10shows that a higher silicon content can make a better performing anode.

Example 10

When 1 mil thick films of polyimide are pyrolized and tested inaccordance with the procedure in Example 1. The reversible capacity andirreversible capacity were plotted as a function of the pyrolysistemperature. FIG. 11 indicates that, in one embodiment, it is preferableto pyrolyze polyimide films (Upilex by UBE corp) at around 1175° C.

Additional Examples

FIG. 12 is a photograph of a 4.3 cm×4.3 cm composite anode film withouta metal foil support layer. The composite anode film has a thickness ofabout 30 microns and has a composition of about 15.8% of PI 2611 derivedcarbon, about 10.5% of graphite particles, and about 73.7% of silicon byweight.

FIGS. 13-18 are scanning electron microscope (SEM) micrographs of acomposite anode film. The compositions of the composite anode film wereabout 15.8% of PI 2611 derived carbon, about 10.5% of graphiteparticles, and about 73.7% of silicon by weight. FIGS. 13 and 14 showbefore being cycled (the out-of-focus portion is a bottom portion of theanode and the portion that is in focus is a cleaved edge of thecomposite film). FIGS. 15, 16, and 17 are SEM micrographs of a compositeanode film after being cycled 10 cycles, 10 cycles, and 300 cycles,respectively. The SEM micrographs show that there is not any significantpulverization of the silicon and that the anodes do not have anexcessive layer of solid electrolyte interface/interphase (SEI) built ontop of them after cycling. FIG. 18 are SEM micrographs of cross-sectionsof composite anode films.

II. Electrodes and Electrochemical Cells

As described above, anode electrodes currently used in the rechargeablelithium-ion cells typically have a specific capacity of approximately200 milliamp hours per gram (including the metal foil current collector,conductive additives, and binder material). Graphite, the activematerial used in most lithium ion battery anodes, has a theoreticalenergy density of 372 milliamp hours per gram (mAh/g). In comparison,silicon has a high theoretical capacity of 4200 mAh/g. Silicon, however,swells in excess of 300% upon lithium insertion. When the anode expands,it is often difficult to maintain sufficient adhesion between thesilicon and the current collector. In addition to this, thesilicon-based anode may wrinkle due to the expansion of the anode and/orthe friction between the anode and the other parts of the cell. Thiswrinkling causes the battery to swell in thickness and should be avoidedin order for the battery to have a high volumetric energy density. Thewrinkling also causes the interface between layers (e.g. anodes,cathodes, and separator) to be uneven. As a result, uneven usage of theactive material within a cell and other issues could occur due tononuniform distances between the opposing active materials.

Described herein are certain embodiments of electrodes (e.g., anodes andcathodes), electrochemical cells, and methods of forming electrodes andelectrochemical cells that may include a carbonized polymer. Forexample, mixtures, carbon composite materials, and carbon-siliconcomposite materials described above and in U.S. Patent ApplicationPublication No. 2011/0177393, the entirety of which is herebyincorporated by reference, can be used in certain electrodes, cells, andmethods described below.

The electrode described herein is different from the electrodes used incertain conventional cells in at least the following ways: (1) Theactive material portion is a solid film instead of being a coating thatis coated in a liquid form onto the foil, and (2) the attachmentsubstance is a substance that is not originally included within theactive material solid film before attachment. Certain conventionalelectrode coatings are attached to the current collector foil by abinder such as PVDF which is part of the electrode coating itself. Insome cases, another coating such as carbon is used to stabilize theinterface between the active material coating and the current collector.For example, carbon may be a component of the electrode coating. Also,the material that adheres the coating to the current collector is stillthe PVDF even in the case where there is a carbon coating on the currentcollector.

After the material (e.g., silicon composite material) has been formedinto a shape such as a film, the material can be used in anelectrochemical cell (e.g., battery). In certain embodiments, the filmhas a thickness of about 10 to about 150 microns, and in furtherembodiments, the film has a thickness of about 15 to about 45 microns.

During use of the electrochemical cell, the cell is cycled wherein thesilicon composite material absorbs and desorbs lithium during chargingand discharging of the cell. Since silicon can swell in excess of 300%upon lithium insertion, relatively large volumetric changes can occur inthe electrode during absorption and desorption of lithium. Inembodiments where the silicon composite material is formed into films(e.g., sheets), the increase in volume of the film can result inwrinkling of the film. FIG. 19 is a photograph of an example of a filmwith wrinkles. Wrinkles in the film can result in non-uniform lithiationof the electrochemically active material (e.g., silicon compositematerial). The film may also be coupled or attached to a currentcollector (e.g., copper sheet). Wrinkling of the film can, for example,result in delamination of the film from the current collector and lossof ability to collect electrical current.

Described below are methods of forming the film that results in nowrinkling or substantially no wrinkling of the film during cycling orlithiation. In addition, methods of attaching the film to a currentcollector are described as well as methods of attaching an electrode(e.g., anode and cathode) to a separator. Each of these methods can beused individually or in combination with the other methods to improveperformance of an electrochemical cell.

Electrode Attachment Substance for Adhering a Film of ElectrochemicallyActive Material to a Current Collector

An attachment (e.g., adhesive) substance can be used to couple or adherea film that includes electrochemically active material (e.g., siliconcomposite material) to a current collector (e.g., copper sheet or foil).The electrode attachment substance can adhere the film and currentcollector together to prevent delamination between them. The electrodeattachment substance can be placed or sandwiched between the film andthe current collector to form the electrode. Therefore, the electrodecan include the film, the attachment substance, and the currentcollector. In addition, the electrode can include a film with anelectrochemically active material on both sides of the currentcollector. For example, a first electrode attachment substance can besandwiched between a first film with an electrochemically activematerial and a first side of the current collector, and a secondelectrode attachment substance can be sandwiched between a second filmwith an electrochemically active material and a second side of thecurrent collector.

The film may include porosity such as discussed above. Embodiments mayinclude porosity of about 1% to about 70% or about 5% to about 50% byvolume porosity. The electrode attachment substance may at leastpartially be absorbed into the porosity such that at least some of theelectrode attachment substance is within the porosity of the film.Without being bound by theory, the electrode attachment substance may beabsorbed into the porosity by capillary action. For example, a solutionwith the electrode attachment substance can be absorbed into theporosity, and the solution can be dried, leaving the attachmentsubstance within the porosity of the film. The electrode attachmentsubstance within the porosity of the film can increase the mechanicaldurability of the film. As such, the film can result in a composite filmthat includes the electrode attachment substances. Furthermore, theelectrode attachment substance may extend through the entire thicknessof the film. For example, a substantial portion of the porosity may beopen such that the film is permeable to a solution that includes theelectrode attachment substance. Thus, the electrode attachment substancemay be a continuous phase within the film. In other embodiments, theelectrode attachment substance may only extend partially through or intothe thickness of the film.

In certain embodiments, the electrode attachment substance issubstantially electrically nonconductive (e.g., the electrode attachmentsubstance has an electrically conductivity such that, in use of theadhesive substance in an electrochemical cell, the attachment substancedoes not conduct electricity). Although the electrode attachmentsubstance may be substantially electrically nonconductive, theelectrochemical cell can result in better performance than if theelectrode attachment substance was electrically conductive. Withoutbeing bound by theory, absorption of the electrode attachment substancemay result in portions of the film physically contacting the currentcollector.

The electrode attachment substance may be a polymer. In certainembodiments, the electrode attachment substance includes polyamideimide(PAI) or is PAI. In further embodiments, electrode attachment substanceincludes polyvinylidene fluoride (PVDF) or is PVDF, includescarboxymethyl cellulose (CMC) or is CMC, or includes polyacrylic acid(PAA) or is PAA. The electrode attachment substance may also be othermaterials that provide sufficient adhesion (e.g., bonding strength) toboth the current collector and the film that includes electrochemicallyactive material. Additional examples of chemicals that can be or beincluded in the electrode attachment substance include styrene butadienerubber (SBR), polypyrrole (PPy), poly(vinylidenefluoride)-tetrafluoroethylene-propylene (PVDF-TFE-P), polyacrylonitrile,polytetrafluoroethylene, polyhexafluoropropylene, polyethylene oxide,polypropylene oxide, polyphosphazene, polysiloxane, polyvinyl fluoride,polyvinyl acetate, polyvinyl alcohol, polymethylmethacrylate,polymethacrylic acid, nitrile-butadiene rubber, polystyrene,polycarbonate, and a copolymer of vinylidene fluoride and hexafluoropropylene. The electrode attachment substance may be a thermoset polymeror a thermoplastic polymer, and the polymer may be amorphous,semi-crystalline, or crystalline.

Pressure may be applied to press the current collector and the filmtogether with the electrode attachment substance between. In certainembodiments, significant reduction in wrinkling is achieved whenpressures above 1 bar are applied and better results may occur whenpressure above 2 bars is applied. Pressure can be applied, for example,by putting the film, electrode attachment substance, and currentcollector through rolls such as calendaring rolls.

Another advantage to using an electrode attachment substance between thefilm and the current collector is that the complete assembly can be moreflexible than the film without the current collector and attachmentsubstance. For example, in certain embodiments, the film can be brittleand cannot be deformed (e.g., bent) significantly without cracking andfailure of the film. When the same film is coupled with or attached to acurrent collector with the electrode attachment substance, the completeassembly can be bent or deformed to a further extent compared to a filmthat is not coupled with or attached to a current collector withoutcracking or failure of the film. In certain embodiments, an electrodewhere the minimum radius of curvature before cracking is about 7 mm canbe wrapped around a radius of about 1 mm after the attachment andcalendaring has taken place. Advantageously, the complete electrodeassembly can be rolled to form a rolled-type (e.g., wound) battery.

There are a number of methods of making an electrode with an electrodeattachment substance adhering the film and current collector together.Described below are a number of examples. In certain examples, asolution of an electrode attachment substance and a solvent is made. Forexample, the electrode attachment substance can include PAI and thesolvent can include N-methyl pyrrolidone (NMP). The solution can beapplied or coated onto the current collector and/or the film. In certainembodiments, the coating of solution has a thickness of about 1 μm toabout 100 μm. For example, the coating of solution may have a thicknessof about 50 μm. The film and current collector can then be placed intocontact with one another such that the solution is sandwiched betweenthe film and current collector. As described above, at least some of thesolution may be absorbed into porosity of the film. Since the solutionmay be absorbed into the film, the amount of solution coated onto thecurrent collector or film may vary depending on the thickness andporosity of the film. Excess solution may be blotted using an absorbentmaterial. The solution can then go through one or more drying steps toremove the solvent from the solution leaving the electrode attachmentsub stance.

Another method of forming an electrode with an electrode attachmentsubstance adhering the film to the current collector includes using anelectrode attachment substance that is in a substantially solid stateand heating the electrode attachment substance to adhere the film to thecurrent collector. In some instances, heating can include heatlaminating, e.g., roll pressing or flat pressing, which can allow foreasier manufacturing. The current collector can be a typical currentcollector, for example, a metal foil (e.g., a copper foil). The film caninclude an electrochemically active material. For example, the film caninclude the composite materials described herein, e.g., siliconcomposite materials, carbon composite materials, and/or silicon-carboncomposite materials. Thus, in certain embodiments, the film can includea carbon phase that holds the film together. The film can also includesilicon and can be for an anode or cathode.

In certain embodiments, the electrode attachment substance can bedisposed or sandwiched between the film and the current collector. Forexample, the method of forming an electrode can include providing acurrent collector with a first electrode attachment substance on a firstside of the current collector. The first electrode attachment substancecan be in a substantially solid state. The method also can includedisposing a first solid film comprising electrochemically activematerial on the first electrode attachment substance. Furthermore, themethod can include heating the first electrode attachment substance toadhere the first solid film to the current collector. In someembodiments, heating comprises heat laminating, roll pressing, or flatpressing.

Compared to certain embodiments using a polymer adhesive solutionprocess (e.g., a wet process) to bond films to a current collector, theembodiments using an electrode attachment substance in a substantiallysolid state (e.g., a substantially dry process) can form a more orsubstantially uniform layer of the adhesive between the film and thecurrent collector. For example, the substantially dry process can reducepotential non-uniformity of the adhesive between the film and currentcollector on the micron scale (e.g., reducing possible “columns” ofpolymer adhesive separated by voids on the micron scale). In someexamples, the electrode attachment substance can form a substantiallyuniform layer, for example, of a thermoplastic polymer. Thesubstantially uniform layer can be disposed substantially over an entiresurface of the film. In some embodiments, using an electrode attachmentsubstance that is in a substantially solid state can also reduce thedistribution of adhesive throughout the interior void space (such aswithin the pores) of a composite film. For example, the solid film ofthe electrode described herein can include porosity. In the embodimentsformed by the substantially dry process, a majority of the porosity canbe substantially free of the attachment substance. For example, at leastabout 60%, at least about 65%, at least about 70%, at least about 75%,at least about 80%, at least about 85%, at least about 90%, at leastabout 95%, at least about 97%, at least about 98%, or at least about 99%of the porosity can be substantially free of the attachment substance.

Additionally, using an electrode attachment substance that is in asubstantially solid state can reduce the deposition of adhesive on thesurfaces of the film which are not in contact with the currentcollector. The electrode attachment substance on the film may in somecases, limit the use of some coated separator materials due to possibleincompatibility between the polymer on the separator and the electrodeattachment substance that may be on the film. Thus, certain embodimentscan form an electrode with surfaces of the film substantially free ofthe electrode attachment substance, thereby reducing or substantiallyeliminating the polymer incompatibility issue. In some embodiments,polymers that are not soluble in nonaqueous electrolyte solutions (suchas electrolytes made with carbonate solvents) can be used. For example,polymers such as PAI or PVDF can be used. Furthermore, some solublepolymers may not have enough adhesion strength to maintain electricalcontact during electrochemical testing. By using an electrode attachmentsubstance that is in a substantially solid state, a variety ofnon-soluble polymers (in aqueous or nonaqueous solutions) may also beused. For example, any electrochemically appropriate thermoplasticpolymer, including non-soluble thermoplastic materials such aspolyethylene and polypropylene, can be used.

In some embodiments of formed electrodes, the electrode attachmentsubstance can be substantially electrically nonconductive (e.g., theelectrode attachment substance has an electrically conductivity suchthat, in use of the adhesive substance in an electrochemical cell, theattachment substance does not conduct electricity). Although theelectrode attachment substance may be substantially electricallynonconductive, an electrochemical cell incorporating certain embodimentsof the formed electrode can result in a better performance including,but not limited to, lower irreversible capacity than if the electrodeattachment substance was made electrically conductive. For example, insome embodiments including an electrode attachment substance that iselectrically conductive, conductive particles in the attachmentsubstance may make it harder for the composite material in the solidfilm to contact the current collector. Without being bound by theory,during the heated lamination process, portions of composite material inthe solid film may be able to penetrate the substantially solidelectrode attachment substance and come in direct contact with thecurrent collector, thus allowing the electrons to travel directly fromthe film to the current collector.

Furthermore, certain embodiments of electrochemical cells incorporatingcertain embodiments of the formed electrode can retain good mechanicalintegrity after cycling. For example, during cycling, assemblies maydeteriorate with the composite film delaminating from the currentcollector. Composite films may also flake away from the currentcollector when the assembly bends below a certain radius, which maypreclude winding as a cell assembly method. In certain embodiments, theelectrode assembly can be assembled into a cell using winding.

As described herein, certain embodiments of electrodes formed using asolution process (e.g., a wet process) for attaching the composite filmsto the current collector may be able to be bent to a radius of curvatureof at least 7 mm without significant cracking. Certain embodiments ofelectrodes formed using an attachment substance in a substantially solidstate (e.g., a substantially dry process), can allow for an even smallerbend radius, e.g., to at least about 3 mm, to at least about 2.5 mm, toat least about 2 mm, to at least about 1.5 mm, or to at least about 1mm, without delamination during cycling. Furthermore, the first-chargeirreversible capacity can also be reduced, e.g., from about 15% (using awet process) to about 10% (using a substantially dry process), which caneffectively increase the achievable volumetric energy density.

In some embodiments using a substantially dry process, the adhesion maybe stronger than in some embodiments using a wet process. Without beingbound by the theory, adhesive materials are conveyed by capillary actionin the wet process and are “wicked away” by the porosity in thecomposite material, which result in a weaker lamination. During themanufacturing process, the electrode may undergo a punching process insome embodiments, and the punching itself may actually cause physicaldamage to the edges of the electrodes. The active material located inthe damaged edges may be dislocated from the surface. Thus, some activematerial that has reacted with lithium would get electrically removedfrom the system. As a result, capacity loss may result when lithium isisolated from the rest of the system. On the other hand, the adhesive ina substantially solid process is available to form a stronger bond, thusreduce or eliminate the damages to the edges during electrode punching.By reducing or substantially eliminating the damage to the edges of theelectrodes, the first-charge irreversible capacity can be reduced.

The formed electrode can also include a film with an electrochemicallyactive material on both sides of the current collector. For example, afirst electrode attachment substance can be sandwiched between a firstfilm with an electrochemically active material and a first side of thecurrent collector, and a second electrode attachment substance can besandwiched between a second film with an electrochemically activematerial and a second side of the current collector. Thus, the method ofmaking an electrode also can include providing a second electrodeattachment substance on a second side of the current collector. Thesecond electrode attachment substance can be in a substantially solidstate. The method also can include disposing a second solid filmincluding electrochemically active material on the second electrodeattachment substance. The method further can include heating the secondelectrode attachment substance to adhere the second solid film to thecurrent collector. Heating the first electrode attachment substance andheating the second electrode attachment substance can occursimultaneously or sequentially. In some embodiments, the first electrodeattachment substance and the second electrode attachment substance arechemically the same. In other embodiments, the first and secondelectrode attachment substances are chemically different from eachother.

The current collector coated with a first electrode attachment substancecan be provided in various ways. For example, providing the currentcollector with a first electrode attachment substance can includecoating the current collector with a polymer solution on the first sideof the current collector. The polymer solution can include any of thevarious examples described herein, e.g., solutions including PAI orPVDF. In contrast to some of the methods of allowing absorption of theattachment substance into porosity of the film, various methods offorming an electrode can include drying the polymer solution to form thefirst electrode attachment substance, e.g., in a substantially solidstate.

In embodiments where an electrode attachment substance is provided onboth sides of the current collector, providing the second electrodeattachment substance can include coating the current collector with apolymer solution on the second side of the current collector (e.g., asdescribed in various examples herein). In contrast to some of themethods of allowing absorption of the attachment substance into porosityof the film, various methods can include drying the polymer solution toform the second electrode attachment substance, e.g., in a substantiallysolid state.

As described herein, because the first and/or second electrodeattachment substance can be in a substantially solid state, non-solublepolymers, e.g., non-soluble thermoplastic materials such as polyethyleneor polypropylene, can be used. In certain such embodiments, the polymercan be coated on a current collector using an extrusion process insteadof a solution process. For example, providing a current collector withthe first electrode attachment substance can include providing a polymerresin on the first side of the current collector and extrusion coatingthe polymer resin to form the first electrode attachment substance,e.g., in a substantially solid state. In embodiments where an electrodeattachment substance is provided on both sides of the current collector,providing the second electrode attachment substance can includeproviding a polymer resin on the second side of the current collectorand extrusion coating the polymer resin to form the second electrodeattachment substance, e.g., in a substantially solid state. The type andthickness of the polymer resin can be selected based on the desired endproduct. Other methods of providing a first or second electrodeattachment substance in a substantially solid state can include cold orhot calendaring the current collector with a polymer plate using a rollpress or a flat press. The type and dimension (e.g., thickness) of thepolymer plate can be selected based on the desired end product.Furthermore, utilizing the methods of allowing absorption of theattachment substance into porosity of the film or utilizing the methodsof adhering with an attachment substance in a substantially solid statecan be based on materials and/or design choices.

Polymer Cell Construction

A cell attachment substance can also be used to couple or adhere theelectrode (e.g., anode and cathode) to a separator. The cell attachmentsubstance can adhere the electrode and the separator together to preventdelamination between them. The cell attachment substance can be placedor sandwiched between the electrode and the separator. The cellattachment substance may include any electrode attachment substancesdescribed above. For example, the cell attachment substance may includePVDF or be PVDF, may include PAI or be PAI, or may include or be CMC. Insome embodiments, the method of adhering an electrode to a separator mayinvolve a cell attachment substance in a substantially solid state. Theseparator material would therefore have a melting temperature higherthan that of the polymer in the attachment substance.

Since ions pass through the separator between the anode and cathode, thecell attachment substance also allows ions to pass between the anode andcathode. Therefore, the cell attachment substance can be conductive toions or porous so that ions can pass through the cell attachmentsubstance.

Furthermore, a solution or resin may be made with the cell attachmentsubstance and a solvent and methods of attaching the electrode to theseparator may be similar to that described for attaching the film andcurrent collector with the electrode attachment substance. Describedbelow also are a number of examples.

Methods of Using Pressure to Form Silicon Composite Materials andMethods to Ensure that Cells that Include Silicon Composite Materialsare Kept Free of Wrinkles, Flat, and Thin

In certain embodiments, pressure is applied to the cell during formationof the carbon composite material. A mixture can be cast onto a substrateto form a coating on the substrate. The mixture can then be dried toremove solvents, and the dried mixture (e.g., precursor film) can thenbe cured. The precursor film then goes through pyrolysis to convert theprecursor film to the final composite film (e.g., anode film). In someembodiments, the precursor film is heated to about 900 to 1350° C. Whilethe precursor film is being pyrolysed, pressure can be applied to thefilm. In certain embodiments, a pressure greater than about 2 bars isapplied.

Advantageously, films that are pyrolysed under pressure remain flat andsubstantially wrinkle free during cycling or lithiation. Even withoutthe pressure applied during lithiation, the films can remainsubstantially free of wrinkles. Although, in certain embodiments,pressure may be applied during the initial formation (e.g., firstcharge) and initial cycling of the electrochemical cell as well asduring pyrolysis of the mixture. Without being bound by theory, it isbelieved that the wrinkles form during the first expansion and first fewcycles of the electrode and that there is delamination/wrinkling thatoccurs during that first charge and first few cycles. Applying pressureto prevent delamination may be a reason the pressure prevents wrinklingand swelling.

Examples

The following examples are provided to demonstrate the benefits of theembodiments of electrodes and electrochemical cells. These examples arediscussed for illustrative purposes and should not be construed to limitthe scope of the disclosed embodiments.

Electrode with an Electrode Attachment Substance

Attaching a film with an electrochemically active material to thecurrent collector can be performed by the following example methods.Polyamideimide (PAI) (e.g., Torlon 4000 series from Solvay) is dissolvedin a solvent (N-methylpyrrolidone (NMP), dimethylacetamide (DMAC),etc.). In one example, a solution with 10 w.t. % PAI can be prepared bymixing 10 g of PAI (e.g., Torlon 4000T-HV) with 90 g of NMP in a glassbottle. The solution is mixed until a transparent PAI solution isobtained. The solution may be mixed at room temperature for about 30minutes and then at 150° C. for about another 3 hours. The bottle may becovered by aluminum foil for better dissolving. Furthermore, the mixingcan be done, for example, using a magnetic stirring bar.

An example of assembling an electrode using the PAI solution includesproviding a 40×17 cm piece of copper foil with a thickness of about 10μm. Alcohol such as ethanol or IPA can be applied to a glass table, andthe copper foil can be placed over the alcohol onto the glass table.Pressure can be applied and a kimwipe can be wiped over the copper foilto remove any bubbles and excess alcohol between the copper foil and theglass table. A solution with the attachment substance such as a PAIsolution can then be applied to the copper foil. For example, 4 ml ofthe solution can be applied. A caster with a gap of about 50 μm can beused to form a uniform coating of the solution over the copper foil. Oneor more films with an active material can be placed onto the solution. Alint-free cloth can be used to remove excess adhesive. The copper foilcan then be removed from the glass table.

A second film with an active material can attached to the opposite sideof the copper foil by placing the copper foil with the first film downon a glass table. Then a similar procedure can be used as describedabove to apply the solution with the attachment substance and the secondfilm.

The assembly of the copper foil, solution, and film can be dried forabout two hours at about 110° C. and then dried in a vacuum oven forabout one hour at about 110° C. The drying removes the solvent leavingthe attachment substance between the film and the copper foil. The filmand copper foil can be pressed together during drying. For example, anadhesive loading of about 0.6 mg/cm² was used. Individual electrodes canthen be cut from the dried assembly.

Various attachment substances were used to create electrodes. FIG. 20 isa photograph of an anode without an electrode attachment substance in apressure cell after being cycled. The anode disintegrated almostcompletely. Table I lists a number of types of attachment substances(e.g., polymers) that were tested.

TABLE I Type of Content of polymer polymer (w.t. %) Results HSV-900 1.5,3.0 & 5.0 Depending on the HSV-900 content in solution but relativelyweak attachment Solef 6020 1.5, 3.0, 5.0 & 7.0 Depending on the Solef6020 content in solution but relatively weak attachment (better thanHSV-900) Solef 5130 1.5, 3.0, 5.0 & 7.0 Depending on the Solef 5130content in solution and showing the best attachment among PVDF solutionsSolef 5 (4 w.t. % Solef 5130 Depending on the ratio between Solef5130/PAI and 1 w.t. % PAI) & 5 5130 and PAI content in solution but (3w.t. % Solef 5130 relatively weak attachment, and 2 w.t. % PAI) showinggood performance with Solef 5130/PAI (3/2)

FIG. 21 is a photograph of an anode with Solef 5130 (e.g., PVDF) afterbeing cycled. Although this anode adhered to the current collectorbetter than without an attachment substance, a substantial portion ofthe film with the electrochemically active material disintegrated withcycling. When using solution with low PVDF content, there were no largedifference in attachment according to kind of PVDF and its content

In contrast to PVDF, the PAI attachment substance provided robustadhesion after exposure to electrolyte. FIG. 22 is a photograph of ananode with a PAI attachment substance after being exposed toelectrolyte.

To ensure that PAI did not adversely affect the electrochemicalperformance of the cells, cells were built with PAI, PVDF, and pressure,and subjected to rate characterization tests and long-term cycling atdifferent rates. FIG. 23 is a plot of discharge rate as a function ofcycles. The rate characterization test showed considerable variationwithin each group, but demonstrated that PAI-attached cells have similarrate capability compared to pressure cells, as shown in FIG. 23 . Toomany of the PVDF cells in this group failed, so meaningful conclusionswere not made regarding the difference between the electrochemicalperformance of PAI and PVDF.

FIGS. 24 and 25 are plots of the discharge capacity as percentage of 8thdischarge capacity as a function of cycles, and FIG. 26 is a plot ofdischarge capacity as percentage of 2nd discharge capacity as a functionof cycles. The long term cycling tests showed PAI cells outperformingpressure cells, particularly at low rates, as shown in FIGS. 24 and 25 .The tests also showed little difference between different types of PAI,and a large improvement over PVDF, as shown in FIG. 26 . In summary,PAI-attached anodes are less disintegrated and have better cyclingperformance than anodes attached with any other method tested. Adifferent approach with PVDF could lead to better results withPVDF-attached anodes.

Attaching a film with an electrochemically active material to thecurrent collector can also be performed by the following example method.The example illustrates an example silicon composite electrode forlithium-ion batteries. The example includes two pieces of siliconcomposite material bonded to a current collector using a thermoplasticpolymer in a substantially solid state as an attachment substance. Ingeneral, the example electrode can be produced by coating each side of acurrent collector foil with a thermoplastic polymer. A heat laminationprocess can be used to fix the silicon composite material to each sideof the current collector foil.

Copper Coating

The example method can include providing a piece of copper foil (e.g.,about 40 cm×about 20 cm piece of copper foil with a thickness of about 8μm). The copper foil can be from Fukuda Metal Foil & Powder Co., Ltd.The copper foil can be fixed to a flat glass surface with a few drops ofethyl acetate (e.g., supplied by Sigma Aldrich). Aluminum foil (e.g.,two approximately 45 cm long strips of 25 μm thick aluminum foil andabout 1.5 cm in width) can be used to mask the long edges of the copperfoil. For example, the aluminum foil can be fixed to the copper foilsurface with ethyl acetate. The copper foil surface can be cleaned,e.g., with a few drops of NMP and lint-free wipes. A solution with theattachment substance can be applied to the copper foil. For example,about 8 mL of about 5 wt % PVDF in NMP solution can be dispensed at thetop part of the copper foil. Other solutions, e.g., PAI solution, asdescribed herein can also be used. A caster with a gap of about 6 milfrom the glass can be used to form a uniform coating of the solutionover the copper foil.

The wet coated copper foil can be dried a well-ventilated conventionoven (e.g., for about one hour at about 80° C. The dry coated copperfoil can be removed from the oven. If the attachment substance is to beprovided on both sides of the current collector, the dry coated copperfoil can be placed on a glass with the coated side down. The copper foilcan be fixed to the glass with a piece of tape. The coating process canbe repeated for the second side. For example, aluminum foil (e.g., twoapproximately 45 cm long strips of 25 μm thick aluminum foil and about1.5 cm in width) can be used to mask the long edges of the copper foil.The aluminum foil can be fixed to the copper foil surface with ethylacetate. The copper foil surface can be cleaned, e.g., with a few dropsof NMP and lint-free wipes. A solution with the attachment substance canbe applied to the copper foil. The same or different solution than thatused for the first side can be used. For example, about 8 mL of about 5wt % PVDF in NMP solution can be dispensed at the top part of the copperfoil. Other solutions, e.g., PAI solution, as described herein can alsobe used. A caster with a gap of about 6 mil from the glass can be usedto form a uniform coating of the solution over the copper foil.

The wet coated copper foil can be dried in a convection oven (e.g., forabout one hour at about 80° C. The dry double side coated copper foilcan be removed from the oven. The coated copper foil can be trimmed(e.g., into about 20 cm×about 19 cm sheets). The sheets can be stacked,e.g., separated by lint-free wipes, on a drying rack and dried undervacuum, e.g., at about 100° C. for about 7 hours. The vacuum-driedsheets can be removed from the vacuum oven and trimmed (e.g., about 4cm×about 10 cm pieces, leaving about a 4 cm×about 1.5 cm uncoated regionon each piece).

Heat Lamination

The example method of attaching a film with an electrochemically activematerial to the current collector can further include setting a rollpress machine to a desired temperature (e.g., about 230° C.) andallowing the temperature to stabilize. The method can further includesetting the gap between the rolls (e.g., to about 1.6 mm) and the rollspeed (e.g., to about 0.26 cm/s). The method can include providingrubber pieces (e.g., about 5 cm×about 11 cm silicone rubber with athickness of about 1/32 inch and a durometer scale of 90 A) and shimstock (e.g., about 5 cm×about 11 cm with a thickness of about 3 mil).FIGS. 27A-D are illustrations of an example method of assembling theelectrode stack for heat lamination. For example, FIG. 27A illustratesexample materials for the electrode assembly including two pieces ofsilicone rubber sheets 210, two pieces of steel shim stock 220, twopieces of silicon composite material 230, and a double side coatedcopper foil 240 (e.g., as described herein). FIG. 27B illustrates anexample assembled lamination stack with an offset to show stack order.For example, the double side coated copper foil 240 can be sandwiched bythe two pieces of silicon composite material 230. The two pieces ofsteel shim stock 220 can sandwich the two pieces of silicon compositematerial 230. The two pieces of silicone rubber sheets 210 can sandwichthe two pieces of steel shim stock 220. Thus, the final stack order inthis example is silicone rubber sheet 210/steel shim stock 220/siliconcomposite material 230/coated copper foil 240/silicon composite material230/steel shim stock 220/silicone rubber sheet 210. FIG. 27C illustratesthe relative position of the silicon composite material 230 and thecoated copper foil 240 in the example assembled lamination stack. Forexample, one set of the silicone rubber sheet 210 and steel shim stock220 has been separated from the assembly to reveal the silicon compositematerial 230 and the coated copper foil 240.

The example method of attaching a film with an electrochemically activematerial to the current collector can further include feeding theassembled stack into the roll press machine. The uncoated copper regionof the copper foil 240 can be placed at the leading edge. The stack canbe allowed to cool. The electrode assembly can be separated from the twopieces of steel shim stock 220 and two pieces of silicone rubber sheets210 and inspected. FIG. 27D illustrates an example finished electrodeassembly.

In certain embodiments of attaching a film with an electrochemicallyactive material to a current collector with a lamination process (e.g.,a substantially dry process) when compared to certain embodiments with asolution/wet process, the first-charge irreversible capacity can bereduced. As a result, in such embodiments, the volumetric energy densitycan be increased. Table IIA lists the average irreversible capacity forsample electrode assemblies formed by different methods of attachingcomposite films to the current collector. FIG. 28 shows a bar graphdisplaying the results of Table IIA. For example, in certain embodimentsof attaching a film with an electrochemically active material to acurrent collector with a lamination process (e.g., dry lamination) whencompared to certain embodiments with a solution process (e.g., wetprocess), the first-charge irreversible capacity can be reduced fromabout 15% to about 10%. In subsequent cell test results, a difference ashigh as 9% was measured between wet process PAI and dry laminationK9300.

TABLE IIA average average first average first irreversible dischargecharge Product Description capacity capacity (Ah) capacity (Ah) wetprocess PAI 15.22% 0.102 0.120 dry lamination PAI 12.32% 0.108 0.123 drylamination K9300 9.81% 0.108 0.119 dry lamination S5130 9.84% 0.1080.120

Additional methods of application of an electrode attachment substancewere tested. Table IIB lists methods of forming an electrode with anelectrode attachment substance along with the results. For example,certain embodiments of the method can include wet lamination(“conventional”), wet lamination followed by roll pressing at roomtemperature (“conventional & cold pressing”), wet lamination followed byroll pressing at 130° C. (“conventional & hot pressing”), and/or drylamination (“dry type attachment”). In some embodiments, dry laminationcan also be followed by cold or hot pressing.

TABLE IIB Method type Method details Result Conventional Casting on theglass plate with Partial and weak doctor blade using proper solution.attachment Getting wet one side of Cu foil depending on the on the castsolution. PVDF content in Putting anode on the wet side. solution Dryingat oven for 1 hour. Drying at vacuum oven for 1 hour. ConventionalPreparing Cu foil and anode Better attachment & cold attachment usingconventional than conventional pressing method. Cold calendering methodanode-attached Cu foil sandwiched between two polypropylene (PP) plateusing roll press at room temperature. Conventional Preparing Cu foil andanode Better & hot attachment using conventional attachment thanpressing method. Cold calendering conventional anode-attached Cu foilsandwiched method and/or between two PP plate using conventional & rollpress at temperature cold pressing at 130° C. Dry type Dipping Cu foilinto proper Better attachment attachment PVDF solution. Drying at thanconventional room temperature. method and/or Hot calenderinganode-attached conventional & Cu foil sandwiched cold pressing betweentwo Kapton films using roll press at temperature at 130° C.

Using PAI (polyamideimide) to attach films with electrochemically activematerial (e.g., composite films) to copper foils (e.g., currentcollector) to form electrodes (e.g., anode or cathode) has been shown toprevent delamination of the electrode film from the copper foil. Theelectrode films typically will exhibit some volume expansion in the x-ydirection (e.g., the plane of the film rather than the thicknessdirection of the film) when lithiated (e.g., charged). The dimensionexpansion in plane of silicon composite-based electrode films may bereduced to essentially zero (e.g., about zero) to about 10% whenattachment using PAI is used. In addition to the reduction in x-yexpansion (e.g., in plane expansion), the electrodes (e.g., the assemblyof the film with electrochemically active material attached to thecopper foil) show essentially no failure or at least no significantfailure. For example, failure can include delamination between the filmand the copper foil and/or breakage of the current collector. Therefore,the electrode attachment substance may allow for expansion of the anodeactive material and current collector without significant breakage ofthe current collector and without significant delamination of the filmfrom the current collector. In cases where an inferior adhesion layer isused, the film can be peeled off the copper foil after disassembly of acell that has been charged. In the case of PAI, the electrode assemblyremains intact. Without being bound by theory, it is believed that thePAI attachment is superior to other approaches because of the affinity(e.g., better adhesion) of PAI to the film and the copper foil. PAI hasbeen used in lithium-ion batteries for other applications such as apolymeric binder of a silicon particulate electrode as described by N.Choi et al., “Enhanced electrochemical properties of a Si-based anodeusing an electrochemically active polyamide imide binder,” Journal ofPower Sources 177 (2008) 590-594. In addition to the physical attachmentenhancement, cells built with PAI-attached electrode assemblies haveshown better testing results compared to cells with silicon-containingself-supported electrode films adhered to the current collector using adifferent polymer such as PVDF.

Initial cell discharge rate capability tests show a higher ratecapability compared to cells made with different attachment methods.Table III includes data for the PAI-attached anode assembly cells.Direct control cells made using anode active material films attached tocopper with PVDF had a 2C discharge capacity of about 45%.

TABLE III Type of C/5 C/2 C 2 C Attachment Discharge Discharge DischargeDischarge Used Capacity Capacity Capacity Capacity 20% PAI in NMP 100%96.41% 90.36% 70.12% 15% PAI in NMP 100% 96.18% 89.39% 58.36% 20% PAI inDMAc 100% 96.19% 88.62% 61.86% 15% PAI in DMAc 100% 95.96% 88.69% 53.83%

In addition to discharge capacity, testing has shown an enhancement ofup to three times in cycle life in some cases for the cycle life ofcells built with electrode assemblies attached with PAI versus PVDF. ThePVDF-attached electrode assemblies were superior in performance than theother materials that had been tested other than PAI.

As can be seen in the photograph of FIG. 22 , the anode active materialfilm is attached well to the current collector without any expansionafter cycling and disassembly. In addition, carboxymethyl cellulose(CMC) can be used as an electrode attachment substance that may yieldsimilar results. Other examples of possible attachment substancesinclude polyimide, epoxy, conductive glue, Na-CMC, PAI, etc.Furthermore, treating the copper foil surface (e.g. roughening, plasmatreatment) may further improve adhesion of the film to the copper foil.Describe above are various attachment substances and methods ofattaching the film to the copper foil. Each of the attachment substancesand methods can be used in various combinations.

Electrochemical Cell (Cell Attachment Substance to Adhere Together anElectrode and a Separator)

Attaching an electrode to a separator can be performed by the followingexample methods. A separator coating solution that includes a solventand an attachment substance can be prepared. A mixture can be made ofNMP (630.4 g) and EtOH (157.6 g) with a ratio of 80:20 (other possibleratios are 90:10 to 70:30). PVDF polymer (Solef 6020, 12 g) is added tothe mixture to form a solution. The solution can be mixed at roomtemperature for about 1 hour with magnetic stirrer and then heated toabout 150° C. and mixed until solution is transparent (about 30 mins). Aseparator can be cut to size and held in a fixture during thedipping/coating process. The separator can be any type of polyolefinseparator such as Celgard 2500 and EZ1592. The separator is dipped intothe PVDF solution bath and removed. Excess PVDF solution can be removed,and the separator can be dipped into a water bath for 5 minutes. Theseparator can then be dried for about 4 hours at room temperature andthen dried in a vacuum oven at 60° C. for about 6 hours (about 30 inHg).

The anode dipping solution that includes a solvent and an attachmentsubstance can be prepared. A mixture can be made of acetone (506.3 g),NMP (17.5 g) and EtOH (58.2 g) with a ratio of 87:3:10 (other possibleratios are 85-87:0-3:10). PVDF polymer (Solef 6020, 18 g) is added tothe mixture to form a solution. The solution can be mixed at roomtemperature for about 1 hour with magnetic stirrer and then heated toabout 220° C. and mixed until solution is transparent (about 30 mins).An anode can be held in a fixture during the dipping/coating process.The anode is dipped into the PVDF solution bath and removed. Excess PVDFsolution can be removed. The anode can then be dried in a vacuum oven at110° C. for about 1 hour.

The cell (e.g., pouch cell) can then be assembled using the separatorand anode. The cell can be hot pressed at about 110° C. for about 1 minfor a cell with a thickness of about 1.8 mm and about 2 min for a cellwith a thickness of about 4.5 mm. After hot pressing, the cell is movedimmediately to a cold press at room temperature for a similar time asused with the hot press. A spacer can be used to adjust the gap in thetop and bottom plates of the press to avoid crushing the cell. Thepressing consolidates the PVDF coating on the separator and the PVDFcoating on the anode.

Various cell performance comparisons were made between a polymer cell(e.g., PVDF coatings) and non-polymer cells (e.g., no attachmentsubstance). All the cells used similar anodes. The cells were kept inaluminum clamps and then unclamped. FIG. 29 is a plot of dischargecapacity as a function of cycles. The polymer cells showed nosignificant degradation while the non-polymer cells showed immediatedegradation. FIG. 30 is a plot of discharge capacity as a function ofcycles for two different separators. Separator 1 is Celgard 2500 andSeparator 2 is Celgard EZ1592. The cycle performance did not vary muchbetween the two separator types. FIG. 31 is a plot of discharge capacityas a function of cycles for two different electrolyte solutions.Electrolyte 1 is a EC:DEC:DMC (1:1:1) based electrolyte and Electrolyte2 is a EC:EMC (3:7) based electrolyte. The cycle performance did notvary much between the two electrolytes.

Additional solutions of polymers and solvents were tested. Table IVlists various solutions and results.

TABLE IV Type of polymer content and solution Solution compositionStability Method Results 6 w.t. % Low Two heavy Too thick coating layerHSV-900 in dryers for Broad distribution Ac/EtOH 1 min 10 sec of coatinglayer (90/10) Wrinkles and uneven parts in bottom Partial peel-offproperties between separator and coating layer Bad performance duringfast characterization 6 w.t. % Kynar Low Two heavy Too thick coatinglayer 760 in dryers for Broad distribution of Ac/EtOH(90/10) 1 min 10sec coating layer Wrinkles and uneven parts in bottom Peel-offproperties between separator and coating layer Bad performance duringfast characterization 6 w.t. % K301-F Low Two heavy Too thick coatinglayer in dryers for Broad distribution Ac/EtOH(90/10) 1 min 10 sec ofcoating layer Wrinkles and uneven parts in bottom Bad performance duringfast characterization 4 w.t. % Low Two heavy Relatively thick HSV-900 indryers for coating layer Ac/EtOH 1 min 10 sec Broad distribution (90/10)of coating layer Wrinkles and uneven parts in bottom Peel-off propertiesbetween separator and coating layer Bad performance during fastcharacterization 4 w.t. % K301-F Low Two heavy Relatively thick indryers for coating layer Ac/EtOH(90/10) 1 min 10 sec Broad distributionof coating layer Wrinkles and uneven parts in bottom 3 w.t. % Low Twoheavy Relatively thick HSV-900 dryers for coating layer in Ac/EtOH 1 min10 sec Broad distribution (90/10) of coating layer Wrinkles and unevenparts in bottom Bad performance during fast characterization 3 w.t. %K301-F Low Two heavy Relatively thick in dryers for coating layerAc/EtOH(90/10) 1 min 10 sec Broad distribution of coating layer Wrinklesand uneven parts in bottom Bad performance during fast characterization2 w.t. % Low Two heavy Too thin coating layer HSV-900 dryers for Broaddistribution in Ac/EtOH 1 min 10 sec of coating layer (90/10) Wrinklesand uneven parts in bottom Bad performance during fast characterization2 w.t. % K301-F Low Two heavy Too thin coating layer in dryers for Broaddistribution Ac/EtOH(90/10) 1 min 10 sec of coating layer Wrinkles anduneven parts in bottom Bad performance during fast characterization 4w.t. % Solef Low 1st generation Good coating layer 6020 in air blade forbut still show broad Ac/EtOH(90/10) cold drying distribution of (10 sec)and coating layer heavy dryer Wrinkles and uneven for hot drying partsin bottom (1 min) Better than Heavy dryer only system Moderateperformance during fast characterization 5 w.t. % Solef Low 1stgeneration Good coating layer 6020 in air blade for but still show broadAc/EtOH(90/10) cold drying distribution of (10 sec) and coating layerheavy dryer Wrinkles and uneven for hot drying parts in bottom (1 min)Better than Heavy dryer only system Moderate performance during fastcharacterization 3 wt. % Solef Low 2nd Moderate coating layer but 5130in generation easy to peel-off property Ac/EtOH (80/20) air blade forWrinkles and uneven cold drying parts in bottom (10 sec) and Better than1st heavy dryer generation air blade system for hot drying Moderateperformance (1 min) during fast characterization 3 wt. % Solef Moderate2nd Moderate coating layer 6020 in generation air but easy to peel-offAc/NMP/EtOH blade for cold property (88/2/10) drying Wrinkles and uneven(10 sec) and parts in bottom heavy dryer Better than 1st generation forhot drying air blade system (1 min) Moderate performance during fastcharacterization 3 wt. % Solef Good 2nd Good coating layer 6020 ingeneration Still having wrinkles and Ac/NMP/EtOH air blade for unevenparts in bottom (87/3/10) cold drying Better than 1st generation (10sec) and air blade system heavy dryer Moderate performance for hotdrying during fast (1 min) characterization 3 wt. % Solef Low 2nd Goodcoating layer 6020/Solef 5130 generation Still having wrinkles and(75/25) in air blade for uneven parts in bottom Ac/EtOH (9/1) colddrying Better than 1st generation (10 sec) and air blade system heavydryer Moderate performance for hot drying during fast (1 min)characterization 3 wt. % Solef Moderate 2nd Good coating layer6020/Solef 5130 generation less wrinkles and uneven (75/25) in air bladefor parts compared Ac/NMP/EtOH cold drying with Ac/EtOH and/or (88/2/10)(10 sec) and Ac/NMP/EtOH(88/2/10) heavy dryer Better than 1st for hotdrying generation air blade system (1 min) Moderate performance duringfast characterization 2 wt. % Solef Good 2nd Good coating layer6020/Solef 5130 generation but too thin (75/25) in air blade for lesswrinkles and Ac/NMP/EtOH cold drying uneven parts compared (88/3/10) (10sec) and with Ac/EtOH and/or heavy dryer Ac/NMP/EtOH(88/2/10) for hotdrying Better than 1st generation (1 min) air blade system Badperformance during fast characterization 2 wt. % Solef Good 2nd Goodcoating layer 6020/Solef 5130 generation but too thin (50/50) in airblade for less wrinkles and uneven Ac/NMP/EtOH cold drying partscompared (87/3/10) (10 sec) and with Ac/EtOH and/or heavy dryerAc/NMP/EtOH(88/2/10) for hot drying Better than 1st generation (1 min)air blade system Bad performance during fast characterization 2 wt. %Solef Good 2nd Good coating layer 6020/Solef 5130 generation but toothin (25/75) in air blade for less wrinkles and uneven Ac/NMP/EtOH colddrying parts compared (87/3/10) (10 sec) and with Ac/EtOH and/or heavydryer Ac/NMP/EtOH(88/2/10) for hot drying Better than 1st generation (1min) air blade system Bad performance during fast characterization

Various pressures and heat were tested after cell assembly andelectrolyte addition. This pressure and heat is what seals the coatingson the anode and separator together. Table V lists various pressingprocesses and the results.

TABLE V Temperature conditions during pressing Other conditions Results120 C. for hot press 30 sec for single layer Moderate adhesive propertywithout cell between anode and separator cold press step 2 min for 15layer cell and/or cathode and separator But, bad cell performance 120 C.for hot 30 sec for single layer Moderate adhesive property pressing &cell between anode and separator cold press with 2 min for 15 layer celland/or cathode and separator weight With moderate cell performance Coldpressing 30 sec for single layer Bad adhesive property cell betweenanode and separator and/or cathode and separator And, bad cellperformance Hot pressing & cold 30 sec for single layer Good adhesiveproperty pressing with cell between anode and separator 1 min for 5layer cell and/or cathode and separator 2 min for 15 layer cell withoutkind of cells With moderate cell performance 1 min for single layer Goodadhesive property cell: severe time between anode and separator frame toand/or cathode and separator check damage of But, bad cell performanceseparator 1 min for 5 layer cell Good adhesive property with the highestbetween anode and separator pressure to check and/or cathode andseparator damage of But, bad cell performance separator

A polymer cell was also assembled by attaching electrodes to apolymer-coated separator that had not been fully dried (still hassolvents). Attachment between the polymer coated separator andanode-attached Cu foil or cathode was achieved by directly addinganode-attached Cu foil or cathode and adhering the electrode to thesurface of a solvated PVDF-coated separator. Good attachment wasobserved between the separator and anode and/or separator and cathodebut poor cell performance was measured. Without being bound by theory,the poor performance was likely due to excess PVDF that filled the poresin the separator and electrode.

Other types of coating methods can be used such as spray, electro-spray,electrospinning, casting with doctor blade, etc. Other types ofseparators can also be used such as metal oxide-coated or metal oxidefilled separator.

Pressure Methods to Prevent Anode Wrinkling in Silicon-Based Anodes

Upon lithiation, the silicon in the anode swells and expands. Since thematerial in the electrode is confined between a current collector andthe separator this swelling results in a wrinkly anode. FIG. 32 is aphotograph of an example wrinkly anode.

If the anode is left to expand freely, for example in a beaker cell, thematerial in the anode does not wrinkle, but instead it bulges outward.In this state, it is possible to measure the swelling of the anode layerin the vertical, which in anodes described herein increases from about33 microns (in the pristine state) to about 45 microns (after fullcharge).

The approach used to overcome the wrinkling problem can be twofold: theapplication of pressure to constrain the anode while preventing itslateral expansion, and the use of a blend of polymers to bond anode,cathode and separator together while allowing for movement of thelithium ions. Certain bonding materials are used in some commerciallyavailable polymer cells.

In previous attempts, in order to apply pressure onto the cell, the mainmechanism used was to sandwich the cell between two sheets ofpolypropylene, ⅛ of an inch thick, with green felt as interfacialmaterial between them. The stack was held together with paper clips fromthe sides. This pressure mechanism did not prevent the formation ofwrinkles.

In order to apply a higher pressure during the charging step, othersetups were tested. Several combinations of materials (polypropylene andaluminum) and interface materials (green felt and rubber) were used toeliminate anode wrinkling during cycling. Table VI summarizes theresults.

TABLE VI Interface Pressure Plate material material method Wrinkles Cellnumber Polypropylene Rubber Binder clips YES ML4209-4212 Aluminum Greenfelt C-clamps NO ML4205-4208 Aluminum Rubber C-clamps NO ML4221-4224Aluminum None C-clamps NO ML4237-4240

At high pressures, both green felt and rubber work well. It is importantto note that the force applied with the C-clamps is not very wellcontrolled, as it depends on the manual ability of the operator. In caseof the cells what were clamped without interfacial material (barealuminum), the cell stack was very tightly compressed, and theelectrodes did not even soak with electrolyte. This result is attributedto excess pressure being applied to the cell stack.

In terms of rate performance, the polymer cells constrained withc-clamps show a better performance than past polymer cells, assummarized in Table VII (values shown are the averages for the fourtested cells of each type).

TABLE VII Cap.@C/ Cap.@C/ Cap.@2 C Cell type 5(mAh) 2(mAh) Cap.@C(mAh)(mAh) PP/Rubber/ 386.7257 276.17 138.684 88.65451 binder clips Al/greenfelt/ 472.9861 459.085 443.9014 397.8245 c-clamps Al/rubber/ 464.397458.62 444.6271 387.7365 c-clamps

In order to better understand the amount of pressure needed to preventwrinkling, different weights were applied to 5-layer cells sandwichedbetween aluminum plates (3×3 inch) with rubber pads (2×2 inch). Fourdata points were collected, with just one cell for each weight (25, 50,75 and 100 lb). Of those, only the cell with 100 pounds of force showedcomplete reduction of the wrinkling problem. FIGS. 33A-C are photographsof anodes for weights 100 lb, 75 lb, and 50 lb, respectively.

It is also worth mentioning that applying pressure during forming andthen moving the cell into a lower pressure device seems to be aneffective way to control wrinkling in the anode. This was done by usingthe Al/rubber/c-clamp setup during the first charge and discharge, andthen transfer the cell stack to a Polypropylene/green felt/binder clipssystem. FIG. 34 is a photograph of an anode which shows the absence ofwrinkles when this procedure is followed.

Various embodiments have been described above. Although the inventionhas been described with reference to these specific embodiments, thedescriptions are intended to be illustrative and are not intended to belimiting. Various modifications and applications may occur to thoseskilled in the art without departing from the true spirit and scope ofthe invention as defined in the appended claims.

What is claimed is:
 1. A method of forming an electrochemical cell, themethod comprising: providing an electrode comprising an active materialon a current collector; providing a separator; sandwiching an attachmentsubstance between the electrode and the separator, wherein theattachment substance is in a substantially solid state; andsubsequently, heating the attachment substance to adhere the separatorto the electrode.
 2. The method of claim 1, comprising coating theelectrode with the attachment substance in the substantially solidstate.
 3. The method of claim 1, comprising: coating the electrode witha polymer solution; and drying the polymer solution to form theattachment substance in the substantially solid state.
 4. The method ofclaim 1, comprising coating the separator with the attachment substancein the substantially solid state.
 5. The method of claim 1, comprising:coating the separator with a polymer solution; and drying the polymersolution to form the attachment substance in the substantially solidstate.
 6. The method of claim 1, further comprising applying pressure tothe attachment substance to adhere the separator to the electrode. 7.The method of claim 1, wherein the attachment substance comprisespolyamideimide, polyvinylidene fluoride, carboxymethyl cellulose,polyacrylic acid, polyethylene, or polypropylene.
 8. The method of claim1, wherein the attachment substance is configured to allow ions to passtherethrough.
 9. The method of claim 1, wherein the attachment substancecomprises porosity.
 10. The method of claim 1, wherein the separator hasa higher melting temperature than the attachment substance.
 11. Themethod of claim 1, wherein the separator comprises porosity.
 12. Themethod of claim 1, wherein a carbon phase holds the active materialtogether.
 13. The method of claim 12, wherein the active materialcomprises silicon particles distributed within the carbon phase.
 14. Themethod of claim 13, wherein the carbon phase comprises hard carbon. 15.A method, comprising: forming an attachment substance between aseparator and an electrode of a electrochemical cell; and after theforming, heating the attachment substance to adhere the separator to theelectrode.
 16. The method of claim 15, wherein the attachment substanceis in a substantially solid state prior to the heating.
 17. The methodof claim 15, wherein the forming comprises coating the electrode withthe attachment substance in a substantially solid state.
 18. The methodof claim 15, wherein the forming comprises: coating the electrode with apolymer solution; and drying the polymer solution to form the attachmentsubstance in a substantially solid state.
 19. The method of claim 15,wherein the forming comprises coating the separator with the attachmentsubstance in a substantially solid state.
 20. The method of claim 15,wherein the forming comprises: coating the separator with a polymersolution; and drying the polymer solution to form the attachmentsubstance in a substantially solid state.
 21. The method of claim 15,further comprising applying pressure to a stack comprising the separatorand the electrode to adhere the separator to the electrode.
 22. Themethod of claim 15, wherein the attachment substance comprisespolyamideimide, polyvinylidene fluoride, carboxymethyl cellulose,polyacrylic acid, polyethylene, or polypropylene.
 23. The method ofclaim 15, wherein a porosity of the attachment substance allows ions topass between the electrode and the separator.
 24. The method of claim15, wherein the separator has a higher melting temperature than theattachment substance.
 25. The method of claim 15, wherein a porosity ofthe separator permits ions to pass through the separator.
 26. The methodof claim 15, wherein: the electrode comprises an active material, and acarbon phase of the active material holds the active material together.27. The method of claim 26, wherein the active material comprisessilicon particles distributed within the carbon phase.
 28. The method ofclaim 27, wherein the carbon phase comprises hard carbon.